Stool osmolality

Q: By default, stool osmolality is equal to plasma osmolality.

A) True

B) False

Answer: A

Normal fecal fluid's osmolality is about equal to the plasma osmolality of 290 mOsm/kg. The colon cannot dilute stool to an osmolality lower than that of plasma. 

This simple piece of information has immense clinical implications. 

If stool osmolality is less than 250 mOsm/kg, it suggests factitious diarrhea or the addition of water or a hypotonic solution. Frequently, urine is used for this purpose, which is easy to do, but urine is a hypotonic solution.

It is worth noting that stool osmolality also increases due to the breakdown of carbohydrates by bacteria into smaller, osmotically active molecules.

#GI

#electrolytes

References:

1. Thomas PD, Forbes A, Green J, et al. Guidelines for the investigation of chronic diarrhoea, 2nd edition. Gut 2003; 52 Suppl 5:v1.

2. Topazian M, Binder HJ. Brief report: factitious diarrhea detected by measurement of stool osmolality. N Engl J Med 1994; 330:1418.

3. Pollock RC, Banks MR, Fairclough PD, Farthing MJ. Dilutional diarrhea: underdiagnosed and over-investigated. Eur J Gastroenterol Hepatol 2006; 12:609.